1. Field of the Invention
The present invention relates to a heteropolyacid catalyst including rholybdenum (Mo) and phosphorus (P) for use in producing unsaturated acids such as acrylic acid, methacrylic acid or the like in the gas-phase catalytic oxidation of unsaturated aldehydes such as acrolein, methacrolein, or the like or saturated aldehydes such as isobutyraldehyde, where the catalyst is prepared using a nitric acid to molybdenum mole ratio of at least 0.5:1 or a nitric acid to Mo12 ratio of at least 6.0:1 during catalyst preparation and/or the catalyst has at least 50% medium pores, preferably at least 57% medium pores, and method for making and using same.
More particularly, the present invention relates to a heteropolyacid catalyst including molybdenum (Mo), phosphorus (P), vanadium (V), bismuth (Bi), and optionally, copper (Cu), an optional first component selected from the group consisting of potassium (K), rubidium (Rb), cesium (Cs), thallium (Tl), or mixtures or combinations thereof, and an optional second component selected from the group consisting of antimony (Sb), boron (B), tungsten (W), cerium (Ce), niobium (Nb), indium (In), iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), manganese (Mn), arsenic (As), silver (Ag), zinc (Zn), lead (Pb), tin (Sn), titanium (Ti), aluminum (Al), silicon (Si), tantalum (Ta), germanium (Ge), gallium (Ga), zirconium (Zr), magnesium (Mg), barium (Ba), lanthanum (La), or mixtures or combinations thereof, used in producing unsaturated acids such as acrylic acid, methacrylic acid or the like by the gas-phase catalytic oxidation of unsaturated aldehydes such as acrolein, methacrolein, or the like or saturated aldehydes such as isobutyraldehyde, where the catalyst is prepared using a nitric acid to molybdenum mole ratio of at least 0.5:1 or a nitric acid to Mo12 ratio of at least 6.0:1 during catalyst preparation and/or the catalyst has a pore size distribution comprising at least 50% medium pores, preferably at least 57% medium pores, and method for making and using same.
2. Description of the Related Art
Many, if not all, prior art heteropolyacid catalysts for the production of unsaturated acids from unsaturated aldehydes have taught the use of nitric acid or other strong mineral acids in the preparation of the heteropolyacid catalyst, while certain prior art patents have taught a critical nitric acid (HNO3) concentration for the preparation of heteropolyacid catalysts, especially heteropolyacid catalysts including bismuth as one of the catalyst components. U.S. Pat. No. 5,102,846 discloses a catalyst preparation using a critical amount of HNO3 to form a heteropoly acid with improved activity and to dissolve bismuth nitrate when present, while U.S. Pat. No. 5,102,847 discloses a catalyst made by using at least one bismuth compound selected from the group consisting of bismuth nitrate and bismuth oxide as a source of bismuth and using nitric acid of more than 1 mole to not more than 5 moles based on 12 moles of molybdenum atoms for dissolving the bismuth compound(s). U.S. Pat. No. 6,624,326 discloses a process for producing methacrylic acid through vapor phase oxidation or vapor phase oxydehydrogenation of at least one of methacrolein, isobutyl aldehyde and isobutyric acid in the presence of a heteropolyacid catalyst containing a heteropolyacid composed of at least one of molybdophosphoric acid and molybdovanadophosphoric acid or a salt of the heteropolyacid, characterized in that said heteropolyacid catalyst has been prepared by a method comprising preparing an aqueous solution or aqueous dispersion which (1) contains the nitrogen-containing heterocyclic compound, nitrate anions and ammonium ions, (2) the ammonium ion content not exceeding 1.7 moles per mol of the nitrate anion content, and (3) the ammonium ion content not exceeding 10 moles per 12 moles of the molybdenum atom content, by mixing raw materials containing the catalyst-constituting elements with the nitrogen-containing heterocyclic compound in the presence of water, drying and calcining the same.
Although several methods have been disclosed for preparing catalysts with critical amounts of nitric acid for the formation of heteropoly acid catalysts, especially catalysts containing bismuth, there is still a need in the art for improved catalysts for the gas phase oxidation of unsaturated aldehydes such as acrolein and methacrolein or saturated aldehydes such as isobutyraldehyde to unsaturated acids, such as acrylic acid or methacrylic acid.